Fuel consumption on-board computer vs reality.

[piotreus]

Hi guys I noticed that the on board computer on my w204 model T om646 c200 shows false value of fuel consumption. One it shows 7.4L/100km but when I went to petrol station I tank 61 ON LITTERS which was 8.3 L/100km. Today I drove 222 km and the computer shows 5.9L but it burn exactly 15.51L which was 6.9L. Despite the fact that consumption is a little to high in my opinion the different between the on board computer and reality is quite weird. Once its 1.9l once it is 1l. Do you have any idea why is it that? I understand that on board computer can missing the really consumption but the difference value should be the same all the time.

 

[Stratman]

How do you know you filled the tank to exactly the same level each time? Pump overflow cut-outs vary in their sensitivity, from tripping at barely more than a trickle to wet shoes.

 

[zipdip]

Well I have always doubted these onboard fuel numbers,I always use this method to work out the MPG ,go to your filling station,fill the car up,with the nozzle of the pump all the way into the fuel tank,when the trigger clicks off stop filling reset your trip and then next time you fill up do the same and then do the calculations using the mileage on the trip.

 

[Jigglyjohnson]

To compare fill your tank at one fuel station and use this station for the test. Fill the tank till it clicks out once. Get the receipt and right down the mileage and at that point reset the trip computer. When the tank is nearly empty of at a point you want to re fill use the same station and refill to again one click and get the receipt. From there you can work out the mileage you have covered from the trip computer and the amount of fuel you have had to refill with( the fuel you have used) then you can work out the average mpg against the trip computer and see what the difference is ??

 

[piotreus]

always refuel at the same station, besides, what could be the difference in the amount of fuel refilled depending on the dispenser? aprox. 0.5l mayby less. 0.5l by 700 km is a amount that doesnt really metters.

i did it just in the way you wrote.
have you checked yours?

 

[leef44]

I think it could be a number of variables. When I first got my SLK, I found I was getting 26 mpg when the trip computer says 28 mpg.

In the last year or so, I have changed fuel station from my local Shell (V Power) to BP (Ultimate) given my local BP station has now brought their prices to a competitive level and it is nearer. I have also had new tyres and the car was serviced just before changing fuel.

Now I get 28 mpg and matches the trip computer. Because all three things changed around the same time, I do not know whether it is a factor of all three or just one or two factors.

 

[Merc220cls]

I always fill my tanks to the brim sometimes using the "self cut out" several times then calculate my consumption manually but compare it with the onboard readings. I also use an XL spreadsheet to record the figures.
The difference between both figures is fairly constant but I've found the readings from my SL is very close, I mean within a few points to the manual figures whereas with my CLS the figures are always at least 2 mpg of a difference.

 

[michaelnaughton]

Hi guys I noticed that the on board computer on my w204 model T om646 c200 shows false value of fuel consumption. One it shows 7.4L/100km but when I went to petrol station I tank 61 ON LITTERS which was 8.3 L/100km. Today I drove 222 km and the computer shows 5.9L but it burn exactly 15.51L which was 6.9L. Despite the fact that consumption is a little to high in my opinion the different between the on board computer and reality is quite weird. Once its 1.9l once it is 1l. Do you have any idea why is it that? I understand that on board computer can missing the really consumption but the difference value should be the same all the time.

.

 

[zipdip]

always refuel at the same station, besides, what could be the difference in the amount of fuel refilled depending on the dispenser? aprox. 0.5l mayby less. 0.5l by 700 km is a amount that doesnt really metters.

i did it just in the way you wrote.
have you checked yours?

Well you want it to be accurate,so it is best to use the same filling station and the same pump,yes you are right the chance that one pump will be a different fill than the other is remote,but by using the same pump removes that,as always in these threads the how to fill your tank up raises it's head,I would hope most drivers have noticed that the pumps all over the place have the same nozzle,and inderneath the bend in the nozzle there are two raised bits,when people design a fuel tank for a car they have that standard to work too,and create a air space at the top of the tank,anyway the correct way to fill is to push the nozzle all the way into the tank when it clicks off remove it to the second ridge and stop filling when it clicks again as the tank is full as it was designed,in that way you can as explained before work out just what your car is doing to the litre/gallon.

 

[190]

My trip computer is reasonably accurate compared to measured fill, usually withing 1 or 2 mpg. It's probably at it's worst on very short trips as the MPG readout seems to drop a set amount every time the engine is stopped and re-started.

My annual mileage is so low that I rarely fill to the brim. 600+ miles from full to reserve is not often something I need.

What I do instead is use the reserve light as a benchmark. I put in a set amount, typically 30L and note the mileage when the reserve light comes on again. I think that is as least as accurate as a fill to the brim and in any case when you do this for years on end any error is insignificant.

Occasionally filling to the brim does have it's uses though - My last fill was 6 weeks ago at £1,47 for E10.

 

[m80]

Just a perception but I've come to believe that a heavy foot creates a greater disparity between OBD and calculated.
If I'm driving a reasonable distance in either the 639 or the 211, and being gentle, I often see the calculated being a little more than the OBD.